Agree with bmike - TimeMachine is your best bet for an integrated system backup solution. If you're only looking to manually back up certain files, then a free solution like DropBox might work for you.

While I agree that TimeMachine is an excellent tool (and I use it) I prefer SuperDuper! or Carbon Copy Cloner. They make a complete, bootable clone of your entire hard disk.

If anything happens to your computer and you have a complete backup (everything including system and all files in bootable form) you can get a new computer, boot from the backup and be back in business in very short order.

If you're serious about this get a decent 7200 rpm firewire 800 external HD and back up your entire computer every day. Both SD and CCC have smart backup features so you're just backing up the differences, not the entire HD again, so, while the first backup takes some time, subsequent backups are quite fast.

This setup has saved my bacon a few times and I've been backing up like this for many years. TimeMachine has a wonderful UI for finding individual files you may have lost and I back up with it as well, but I also make a bootable clone of my working computer every day and while no setup is fail safe I feel pretty secure with this process.